This patch is part of a series that adds origin-tracking to the debugify
source location coverage checks, allowing us to report symbolized stack
traces of the point where missing source locations appear.

This patch adds the logic for collecting stack traces in DebugLoc
instances. We do not symbolize the stack traces in this patch - that
only happens when we decide to actually print them, which will be the
responsibility of debugify. The collection happens in the constructor of
a DebugLoc that has neither a valid location nor an annotation; we also
collect an extra stack trace every time we call setDebugLoc, as
sometimes the more interesting point is not where the DebugLoc was
constructed, but where it was applied to an instruction. This takes the
form of a getCopied() method on DebugLoc, which is the identity function
in normal builds, but adds an extra stack trace in origin-tracking
builds.

Part of the coverage-tracking feature, following #107279.

In order for DebugLoc coverage testing to work, we firstly have to set
annotations for intentionally-empty DebugLocs, and secondly we have to
ensure that we do not drop these annotations as we propagate DebugLocs
throughout compilation. As the annotations exist as part of the DebugLoc
class, and not the underlying DILocation, they will not survive a
DebugLoc->DILocation->DebugLoc roundtrip. Therefore this patch modifies
a number of places in the compiler to propagate DebugLocs directly
rather than via the underlying DILocation. This has no effect on the
output of normal builds; it only ensures that during coverage builds, we
do not drop incorrectly annotations and therefore create false
positives.

The bulk of these changes are in replacing
DILocation::getMergedLocation(s) with a DebugLoc equivalent, and in
changing the IRBuilder to store a DebugLoc directly rather than storing
DILocations in its general Metadata array. We also use a new function,
`DebugLoc::orElse`, which selects the "best" DebugLoc out of a pair
(valid location > annotated > empty), preferring the current DebugLoc on
a tie - this encapsulates the existing behaviour at a few sites where we
_may_ assign a DebugLoc to an existing instruction, while extending the
logic to handle annotation DebugLocs at the same time.

Add per-property has<Prop>/set<Prop>/reset<Prop> functions to
MachineFunctionProperties.

Assertion failure introduced in #140063, which didn't account for TBB
and FBB being the same block.

branch-folder hoists common instructions from TBB and FBB into their
pred. Without this patch it achieves this by splicing the instructions from TBB
and deleting the common ones in FBB. That moves the debug locations and debug
instructions from TBB into the pred without modification, which is not
ideal. The merged instructions should get merged debug locations for debugging
and PGO purposes, which is handled in this patch. Debug instructions also need
to be handled differently. That'll come in another patch. This issue was found
by @omern1.

EnableTailMerge is false by default and is handled by the pass builder.
Passes are independent of target pipeline options.

This completes the generic `MachineLateOptimization` passes for the NPM
pipeline.

Update some other places to avoid implicit conversions this introduces,
but I probably missed some.

Fixes the "use after poison" issue introduced by #121516 (see
<https://github.com/llvm/llvm-project/pull/121516#issuecomment-2585912395>).

The root cause of this issue is that #121516 introduced "Called Global"
information for call instructions modeling how "Call Site" info is
stored in the machine function, HOWEVER it didn't copy the
copy/move/erase operations for call site information.

The fix is to rename and update the existing copy/move/erase functions
so they also take care of Called Global info.